Isothermal production of uniaxially textured YBCO superconductors using constitutional gradients

PHYSlCA ELSEVIER

Physica C 275 (1997) 205 - 210

Isothermal production of uniaxially textured YBCO superconductors using constitutional gradients G.J. Schmitz *, O. Kugeler ACCESS e.V., lntzestrasse 5, D-52072 Aachen, Germany Received 31 October 1996

Abstract

Uniaxially textured samples of bulk (RE)BaCuO were melt-grown in an isothermal furnace utilizing the influence of different rare earth compounds on the peritectic temperature of (RE)BaCuO. A suitable geometric arrangement of different (RE)Ba2Cu30 x compounds during heat treatment leads to a specific diffusion profile of the respective RE elements. According to the distinct peritectic temperatures of the different RE-systems, a monotonous decrease in peritectic temperature could be established. This gradient determines the direction of crystallization during isothermal cooling and moreover governs the selection mechanism resulting in preferred c-axis growth. In contrast to texturing samples in external gradients, the use of a layered structure of different (RE)Ba2Cu30 x compounds allows a tailored texturing or textured coating even of complex shaped products and opens up a variety of interesting perspectives for the production of long length wires and perhaps even of large grained bulk material. Keywords: Growth mechanism; peritectic growth; grain selection

1. Introduction

Melt processing is widely accepted to yield the best superconducting properties in bulk specimen of (RE)Ba2Cu30 ~ ceramics, [1]. Texturing of the material in this context has been shown to be one of the key issues to avoid weak links and accordingly to obtain high critical current densities being mandatory for the application of these materials especially in (RE)BaCuO systems. Various methods to influence the texture have been investigated by now - all being based on the use of an external gradient (magnetic field, electric field, temperature field). The most successful processes are based on seeding tech-

* Corresponding author. Fax: + 49 241 38578.

niques in combination with a temperature gradient being shifted through the sample. Such a defined temperature gradient can easily be established in small samples of regular shape. Scaling of the processes to larger sample sizes a n d / o r complex geometrical arrangements, as requested for many applications, however, is difficult because the isotherms in these cases will deviate from the required planar shape. Complex thermal fields to texture samples of irregular geometry in addition can hardly be realized. We adapted a method known from single crystal growth [2] to texture polycrystalline materials. This method is characterized by the use of a constitutional gradient in combination with isothermal cooling. Constitutional gradients in this context are realized by geometrically arranging at least two different materials before or during the heat treatment. Due to

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the different peritectic temperatures in the (RE)Ba2Cu30 x systems it becomes possible to achieve a continuous increase in peritectic temperature with increasing concentration of the higher melting rare earth compound. Since the physical, especially the superconducting, properties of these systems do not differ significantly, this concentration dependence of the peritectic temperature opens up a variety of alternative - isothermal - processing routes, which can be adapted even to a complex sample geometry. Using this concept, the gradient in the peritectic temperature inside the sample determines the direction of solidification during isothermal cooling. The purpose of the present paper is to investigate this concept for polycrystalline specimen of the specific superconducting systems NdBa2Cu307_ x + YBa 2 Cu307_x/Y2BaCuO 5 +YbBa2Cu30.~ and to discuss implications for future processes e.g. for the production of long length wires or magnetic shielding applications.

1050 ° - 11O 0 °

l

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t/

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/.~--

~

~

.~,~- cooling rates (0.6 - 60 K/h)

\

/

E

rapid heating (25 K/min)

/

\ \,

,/ room _ _

_ holding time (1-10 hours)

time

Fig. 1. Heat treatment of the layered samples. Cooling rates have been varied from 0.6 to 60 K / h in the different experiments.

about 850°C at rates ranging from 0.6 K / h up to 60 K / h . The samples then were quenched to room temperature by removing them from the furnace, Fig. 1. Samples were loaded with oxygen (po2 = 1 bar) at a temperature of 550°C for about 50 hours. Metallographic sections of the samples have been prepared and were analyzed by means of polarized optical microscopy, SEM and EDX-spot analysis.

2. Experimental Experiments were carried out with pure Y-123 powder and mixtures of Y123- and Y211-powders in a molar ratio of 5:1 and 5:2, respectively. These precursors were pressed into discs of 10mm in diameter, and 5 - 1 0 mm in height, which were then calcined for 24 h at 950°C in air. Subsequently one surface of the discs was polished and covered with Nd-123 powder. This was most successfully done by sedimentation in acetone. Alternatively, samples were coated using a scalpel under a stereomicroscope creating different geometrical arrangements, like e.g. parallel lines or concentric circles of Nd-123 powder on top of the sample. An MgO single crystal being covered with a layer of Yb-123 was selected as substrate material. Since Yb-123 has a lower melting temperature than Y-123 it remains in the liquid state during the process and accordingly cancels the heterogeneous nucleation effect of the MgO substrate, [3]. The sandwiched samples then were heat treated in air by rapid (approx. 25 K / m i n ) heating to specific temperatures of about 1050°C (below Tp of Nd-123) or 1100°C (above To of Nd-123). Subsequently the samples were slowly cooled down to

3. Results Microscopic inspection of the processed samples reveals a dense polycrystalline microstructure of millimeter sized 123-domains, Fig. 2. The conical respectively pyramidal - shape of the domains indicates, that nucleation of all grains took place near the Y-Nd-123 interface. In samples not being covered entirely with a dense Nd-123 layer, but with a specific geometric structure, the growth of the grains takes place radially, starting from the Nd-123 covered regions. This microstructure is interlaced by planar defects, mainly being oriented parallel to the basal plane of the pyramidal grains. Fine, micrometer-sized 211-particles are distributed all over the 123-matrix. Polarized optical microscopy of oxygenated samples reveals the typical twin patterns, which have been used to determine the crystallographic orientation of the individual grains, [4]. In sections perpendicular to the layered structure, almost all over the sample the twin pattems, corresponding to the trace of a ( 110> direction, are oriented perpendicular resp.

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400um

Fig. 2. Macroscopic cross section of a locally powder-seeded sample (left) and schematic identification of larger individual grains and their crystallographic orientation (right). The arrows indicate the projection of the c-axis.

parallel to the planar defects, indicating that the c-axis o f each grain is in the sectional plane. Therefore it can be stated, that the c-axis essentially is

oriented parallel to the axis of each pyramidal grain. In case of a dense n e o d y m i u m cover layer, a uniaxially textured p o l y c r y s t a l l i n e m i c r o s t r u c t u r e is achieved, Fig. 3. E D X m e a s u r e m e n t s of the processed samples reveal that the rare earth elements have diffused from their original layers into the adjacent layers. Spot analysis along the sample axis reveals concentration gradients of n e o d y m i u m and ytterbium in the 123 layer, Fig. 4.

100

% RE 8O

60

o',, "'..

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o"-.

•

2O

....... .~......... ,

0 0

fit

..

X

X

i 200

x

x ,

x

x i 400

¥

x •-

,

x

x

• i 600

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,

distance from the Nd-123 layer [tim]

Fig. 3. Detailed view of the selection mechanism. Starting from the seed region (top) many grains nucleate. Those grains with their c-axis parallel to the constitutional (RE) gradient are selected and overgrow misoriented grains.

Fig. 4. EDX-measurement of neodymium and ytterbium contents in the Y-123 matrix for a sample heated rapidly to T = 1050°C and then slowly cooled down to 850°C at 0.6 K/h. Note the monotonously decreasing Nd-123 content. The location of the MgO-substrate would be at ~ 1.5 mm.

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Fig. 5. Large single grain being selected. Note the conical shape in the cross section, which in three dimensions corresponds to a pyramidal growth morphology.The crystallographic c-axis, which can be deduced from the orientation of the planar defects, is parallel to the axis of the pyramid. Those grains that seem to appear inside the 123-matrix have their origin above or below the sectional plane.

It can be observed that the orientation of grains originating from the edges of a Nd-123 seed layer is parallel to the direction of the constitutional gradient. Grains appearing inside the 123-matrix have their nucleation center above or below the sectional plane, Fig. 5. Regarding the shape of the grains along the sample axis hints at a specific selection mechanism that must have taken place, causing c-axis oriented grains to spread in the growth direction, others to shrink and disappear, Fig. 3.

uration to temperatures above Tp of Nd-123, leads to the peritectic decomposition of each layer, (RE)123 (RE)211 + liquid, ( R E ) = Yb, Y, Nd, enabling liquid phase diffusion in all layers. This diffusion leads to a smear-out of the initially sharp concentration distributions of the rare earth elements, as observed in the experiments. Heating to temperatures below Tp of Nd-123 limits this diffusion controlled exchange of neodymium but is on the other hand beneficial with respect to solid Nd-123 remaining as epitaxial nucleation site. The resulting constitutional gradients can be related to a gradient in the peritectic temperature across the sample by using experimental data for the peritectic temperature as a function of rare earth content [5]. The average gradient in Tp in a sample of 0.5 cm thickness can be estimated to 240 K / c m . The maximum gradients occurring at the interfacial layers reach up to some 103 K / c m . The constitutional gradient is relatively flat in the middle of the 123 matrix corresponding to minor deviations from the peritectic temperature of YBa2Cu3Ox, Fig. 6. Isothermal cooling causes a plane of adequate growth conditions, i.e. slight undercooling below the local - (RE) concentration dependent - peritectic temperature to move through the sample. The veloc-

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1040

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4. Discussion The experimental results might be explained considering the following scheme: heating above config-

200

300

400

5(X)

distance [iJrn]

Fig. 6. Change in periteetic temperature in one dimension of the sample due to the influence of decreasing neodymium content in an YBCO matrix (calculated with data from Goodilin et al. [5]). Effects of Yb have been neglected.

G.J. Schmitz, O. Kugeler / Physica C 275 (1997) 205-210

ity of that movement is related to the external cooling rate by:

3 ( T - Tp)13t v = o(r-

3T/3t- 3Tpl3t

209

ally controlled growth mechanism. For all arrangements analyzed, the c-directions of the grains correspond to the direction of the gradient of the Nd diffusion field, Fig. 2.

T p ) / O x = Or/Ox - OTp/ax"

which in case of isothermal cooling (aT/ax = O) and with aTp/at = 0 (since Tp being materials property) reduces to 0T

OT

at 0rp

ax

0t -

0c'

ac ax

where Tp is the local peritectic temperature depending on the local molar concentration c(x) of Y, Nd and Yb, respectively. This velocity may be identified with the translation velocity in Bridgman type processes as well as with the velocity of the isotherms in VGF furnaces, w h e r e OTp/OX = 0 and OT/Ox 4=O. The plane enters the sample at the top of the Nd-123 layer. The partially molten Nd-211 +Liquid (now with Y-additions) retransforms to Nd-123 by peritectic reaction, resulting in a polycrystalline structure of multiple, non oriented grains and liquid phase inclusions. Some of the Nd-123 grains serve as nucleation centers for the crystallization of Y-123. Due to the matching lattice geometry epitaxial growth of YNd123 on the Nd-123 crystals takes place requiring lower undercoolings than necessary for heterogeneous nucleation. This may be one reason for most of the grains having their origin in the Nd-123 layer. From these nucleation centers competitive growth leads to selection of the preferred growth orientation of the grains. Such a selection process for preferred c-axis oriented growth has been proposed by Miiller and Freyhardt [6] for local thermal gradients. It becomes effective if the external production velocity is selected smaller than the growth velocity of YBCO in any crystallographic direction. This grain selection concept can readily be transferred to constitutional gradients. As only very low cooling rates were used in our experiments, the production velocity was assured to be always smaller than the growth velocity of YBCO in any direction. Examining samples with geometrical arrangements of Nd-123 powder, differing from a simple layered structure, supports the idea of a constitution-

5. Conclusions and outlook

An isothermal processing method to produce uniaxially textured (RE)Ba2Cu307_ a material has been investigated. Using constitutional gradients in the RE concentration, c-axis oriented thick films can be obtained. The mechanisms leading to this textured microstructure can be explained by adapting a grain selection-model [6] to constitutional gradients. Since the major growth direction is parallel to the c-axis, only scarce segregation occurs at the a/b planes, leading to clean a/b grain boundaries between adjacent grains. The kink angles between the a/b planes of these grains still vary significantly. High critical currents have, however, recently have been reported from bulk YBCO samples exhibiting even a large number of clean, high angle grain boundaries between the individual a/b planes [7]. The isothermal process proposed within this paper in principle offers a variety of possibilities to texture (RE)Ba2Cu307_ a materials and to simultaneously achieve high critical current densities in the respective samples. In contrast to texturing samples in external fields (temperature, magnetic field,...) the use of a layered sandwich structure of different (RE)Ba2Cu307_ compounds allows a tailored texturing or textured coating even of complex shaped products. The coordinated nucleation of the individual grains as well as their coordinated growth along their c-axis will allow for an essentially higher production rate, which can linearly be scaled by the number n of nucleation sites since Uproduction = HUgrowth. If all the n nucleation sites reveal about the same crystallographic orientation, an alternative route to the production of highly textured, high j~ wires of (RE)Ba2CU3OT- a opens up. Many improvements of the proposed process can still be imagined, which mainly refer to the use of different seeds, but especially to the use of technologically relevant substrates:

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The use of platelet like Nd-123 seeds, which can easily be produced from a B a - C u - O rich flux, will decrease the selection length, because during sedimentation these platelets will arrange automatically with their c-axis in the desired direction due to geometrical and mechanical reasons. Post Processing in this context also allows to shorten the selection length by the use of thermal cycling at temperatures close to the peritectic t e m p e r a t u r e of NdBa2Cu307_ ~. This cycling leads to a preferred melting of the smaller grains upon heating and a preferred growth of the larger grains upon cooling, essentially due to the Gibbs-Thomson relation. A more sophisticated mechanical arrangement might be considered to align these seeds even in their a/b direction. Such a process would similar to the process being proposed by Soylu et al. [8], since the growth would be in c-direction avoiding grain boundary segregation at the a/b planes as stated above. Besides mechanical alignment of the seed platelets, in special cases the a/b orientation of these platelets may also be influenced by magnetic fields [9]. To get closer to technical applications one major drawback still has to be overcome - the MgO single crystals, which have been used as substrate materials by now. A possible alternative can be seen in dense BaZrO 3 [ 10,11 ] (produced by e.g. plasma coating technology) acting as an inert buffer layer between the RE-layersandwich and technical substrates like e.g. nickel or Hastelloy. Due to the higher peritectic temperature of the Nd-123 compound a dense layer of this material, not allowing any infiltration by the bariumcuprate melt, could also represent another possible alternative to

the MgO single-crystal substrate. Respective investigations are going on and will be reported in a forthcoming paper. If especially the substrate problem can be solved, an interesting perspective opens up for the production of large area coatings, of long length wires, of coatings of complex shaped specimens and perhaps even of large grained bulk material.

Acknowledgements This work is funded by the German Ministry of Education and Research (BMBF) under grant No. 13 N 6661 4.

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